The only Solution I know is Plop Bootmanager with that you can boot from USB-Devices its pretty good for old pcs. I think you can pack it on to a floppy disk. Put it in your old machine use the bootmanager to chose boot from usb drive. USB drive should be plugged in directly to your mainboard. Here a link : http://www.plop.at/en/bootmanager/index.html
If you have no floppy disk I would say grab an optical drive.